Optical measuring or testing apparatus



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OPTICAL MEASURING OR TESTING APPARATUS Filed June l, 1938 l5 Sheets-Sheet 13 l llllllllllllll 510 l 5" 512 Fig. 49 "mi ,/,I/.nlm l 49 22Std@ STARS .PATENT OPTICAL MEASURING OR TESTING APPARATUS Richard Edmund Reason, Leicester, England, as-

signor to Kapella Limited, Leicester, England, a company of Great Britain Application June 1, 1938, Serial No. 211,231 In Great Britain June 2, 1937 25 Claims.

This invention relatescto optical measuring or testing apparatus and'inore particularly concerned with the measurement or testing of plane section profiles of manufactured articles or of appliances used in their manufacture, such for example as screwthreads or hobs or thread gauges.

Hitherto precision measurement of screwthreads has usually been effected optically by what may be termed shadow projection, by directing a beam of parallel light at an angle to the axis of the thread equal to the pitch angle, so that a shadow of the thread is thrown on to a screen through a magnifying projecting lens. Such an arrangement will often sufce for simple thread surfaces, but is necessarily limited in its practical application owing to the fact that in many instances other parts of the screwthread or other object under examination are interposed in the path of the beam of light.

The primary object of the invention is to provide an improved optical apparatus for effecting precision measurement or testing, which will be of much more general applicability than the known shadow projection arrangement, and will effect the desired measurement with a very high degree of accuracy. This object is generally achieved in the invention by the employment of means for defining a plane section prof-lle of the object with greater accuracy than hitherto in combination with improved means for optically projecting an image of the profile. n

Although the section profile may be defined in other ways, it is preferred to employ for this` purpose an illuminating device by means of which a beam of light bounded (at least on one side) by a plane is directed on to the object under examination, the diffused light from the surface of the object being utilised for the optical projection of an image of the profile of the section in which the object is cut by such. bounding plane. Whilst it is preferable to project the image of the profile in a direction ata-ight angles to the section plane, this will often be impracticable owing to interference from other parts of the object,` and recourse must then be had to oblique projection. In some instances, where only a relatively small degree Ofpbliquity is called for in the projection, satisfactory results can be obtained with the use of a wide angle projecting lens whose optical axis -is perpendicular to the section plane and also to the image plane, but in the majority of instances it will be necessary to employ a projecting lens or lens tion plane, and this in its turn introduces diliiculties in obtaining an image which is both sufficiently accurate and conveniently arranged for eflecting the desired measurements.

A further object of the invention is therefore to provide an oblique projecting arrangement which will give an accurately focussed image and preferably one which will give an image free from the distortion due to the oblique projection.

In some instances it is not possible or convenient to view the whole of a desired section profile from any one viewpoint, and another object ofthe invention is to provide means whereby images of parts of the profile obtained from separate viewpoints can be accurately correlated with one another.

Although it will usually be more convenient to record the image photographically or to project it on to a dilfusing`sc`eif't will sometimes be desired to view the image directly through an eyepiece, and a further object of the invention is so to arrange an oblique projecting device as to facilitate such eyepiece viewing.

For accuracy in definition of the section prole, it is usually preferable to provide an objective in the illuminating device for focussing more or less accurately on the surface of the object an image of a slit or knife edge. It is undesirable, however, to draw on depth of focus to any considerable extentand a further object of the invention is to provide an illuminating device which will accurately dene the prole in cases where the shape of the object is such that the profile diverges very considerably from a straight line.

A more-specic object is to provide an adjustable illuminating device whereby accurately focussed definition of the section profile can be obtained with objects of different shape.

A still further object of the invention is to provide means whereby interference with the accuracy and definition of the projected image resulting from direct reflection of the illuminating light from parts of the object surface is reduced to a minimum.

Further objects of the invention will be apparent from the following description of the accompanying drawings, which illustrate a number of' alternative constructions according to the invention. lIn these drawings- Figure 1 illustrates diagrammatically a simple arrangement in which the section prole is dened by the edge of a shadow cast by a knife .55 edge or mask and is projected obliquely, system whose optical axis is oblique to the sec- Figure 2 shows an alternative arrangement 2 assegna employing slit illumination and also means for correcting for the distortion due to the oblique projection,

Figure 3 is a modification of the arrangement of Figure 1 wherein the section profile is illuminated by focussing a knife edge approximately on the surface of the object,

Figure 4 illustrates means whereby a distorted photographic image obtained with the arrangement of Figure 3 can be reprojected to correct for the distortion,

Figure 5 shows an alternative arrangement employing focussed slit illumination with a projecting lens arranged to give a rectified image,

Figure 6 illustrates a preferred oblique projectinglens system giving a rectified image,

Figures '7 and 8 show modifications of the lens system of Figure 6,

Figures 9-12 respectively illustrate four arrangements for direct viewing of an obliquely projected image of the section profile,

Figures 13 and 14 respectively show two alternative illuminating arrangements for defining the section profile on the two flanks of a gear tooth or screwthread,

Figure l5 illustrates another arrangement for illuminating a portion of a screwthread,

Figure 16 is a partial viewat right angles of Figure 15,

Figures 17 and 18 are two similar Views showing a modification of part of the arrangement of Figures 15 and 16,

Figures 19-21 illustrate a preferred illuminating arrangement adjustable for use with screwthreads or gear teeth of various shapes,

Figures 22-30 illustrate a number of alternative arrangements in which the projecting system of Figure 6 is rotatable to enable the section profile to be viewed from various directions, Figures 28 and 30 respectively being side views of parts of the arrangements of Figures 2'1 and 29 in the direction of the arrows shown,

Figures 3l and 32 show another rotatable projecting arrangement employing the projecting lens of Figure 5, Figure 31 being a section on the line 3 |-3I of Figure 32,

Figures 33 and 34 respectively show two alternative arrangements in which the projecting system of Figure `2 is rotatable,

Figures 35 and 36 respectively illustrate two alternative illuminating arrangements especially intended for testing the pitch of a screwthread,

Figure 87 shows a modification of the arrangement of Figure 36,

Figure 38 illustrates an arrangement for testing the accuracy of meshing of two gear wheels,

Figures 39 and 40 show a simple arrangement utilising polarised light and a modication thereof,

Figures 41-50 illustrate a preferred practical construction of apparatus according to the invention.

In the simple arrangement of Figure 1, the illuminating device comprises a source of light preferably of small size associated with a condensing lens |0|, with or without a concave reector behind the light source, so that a large percentage of the light from the source is col- -lected and concentrated on a small aperture in a mask |02 at the focus of an objective |03. A beam of parallel light is thus transmitted towards the object, which is shown by way of example as a screwthread |04, and a knife edge is interposed in such beam in a position very close to the surface of the object, so as to cast a sharply defined shadow thereon, the edge of the shadow constituting the section plane, indicated at |06.

The diffused light from the surface of the object on the side of the section plane which is illuminated is used for projecting an image of the profile for examination, and the surface of the object may be whitened or otherwise treated to increase the brightness of the projected imgAlthough it would be preferable to project the image in a direction at right angles to the section plane |06, this will seldom be practicable owing to interference from other parts of the object, and Figure 1 shows a simple projecting device for obliquely giving an accurately focussed image of the prole on a iiat screen |01 inclined to the section plane. This device comprises a projecting lens |08, Whose nodal planes respectively intersect the section plane |06 and the plane of the screen |01 in two parallel lines equidistant from the optical axis of the projecting lens. For simplicity in the drawings, the two nodal planes of the lens are assumed to be co-r incident. The image on the screen will be sharply focussed, but will suffer from distortion owing to the oblique projection. Accurate measurements can therefore be made of the image on the screen, in order to test the accuracy of the screwthread, but careful calculations will have to be made from the measurements in order to correct for the distortion and obtain the true measurements.

In practice the amount of light available will usually be inadequate for satisfactory projection on to a screen, and it will consequently often be preferable to photograph the image at low magnication, by replacing the screen |01 by a photographic plate, and either to make the measurements on the photograph or on a photographic enlargement thereof or to project an enlarged image of the photograph on to a screen. A convenient arrangement for this purpose will be described below with reference to Figures 3 and 4.

Figure 2 shows an arrangement employing a modified form of illuminating device and also one way of correcting optically for the distortion due to the oblique projection. rIhe illuminating device of Figure 2 differs from that of Figure 1 in that a narrow slit ||D is provided in the parallel beam from the objective |03 in place of the knife edge |05, the slit being disposed nearer the objective. In order that the illuminating device should not interfere with the projecting device, an inclined reiiector is disposed between the slit ||0 and the object, the object being omitted and the section plane |06 alone indicated, for simplicity.

In this example the projecting device comprises two projecting lenses 2, ||3, of which the first serves to project an accurately focussed but distorted image on to a iiat diffusing surface H4, in a manner similar to that described with reference to Figure l, whilst the second serves to reproject the distorted image at such an angle as to correct for the distortion of the image. In general, the distortion will be properly corrected if the direction of reprojection and the final image plane ||5 are respectively mirror images with respect to the diffusing surface ||4 of the original direction of projection and the section plane |06. Thus in the arrangement illgsgaiedlthe orisiealmsrgiwww" W fltofth..sectiOn,plane:flll6,.,tiiel iwi fusing surface H4 being at .right angles to such plane, whilst the reprojection takes place at 45 to the dilusing surface on the other side thereof, the final image plane ||5 being parallel to or coincident with the section plane. The magnications of the two projecting lenses may be made dilferent by mounting them in the appropriate positions, provided that each satises the condition that its nodal plane passes through the intersection of its object and image planes. The nal image plane ||5 may contain either a projection screen or a photographic plate, as desired.

Figure 3 shows an arrangement which differs from Figure 1 in respect of the illuminating device. In this example an objective serves to focus a knife edge |2| more or less accurately on the surface of the object, the objective preferably being provided with a small aperture |22 which may, if desired, be in the form of a narrow slit. The prole in the section plane |06 is projected in the manner described with reference to Figure 1 on-to a photographic plate I 23. Figure 4 shows a method by which the distortion of the image thus obtained can be corrected by subsequent reprojection of the image at the appropriate angle, and the arrangement is also such as to afford correction for any errors due to lack of flatness in the plate. Thus the image on the plate |23 is reprojected by means of a lens |24 on to a flat screen |25 occupying a position relative to the plate exactly corresponding to that originally occupied by the section plane |06. In order to avoid distortion the plate should be exposed and reprojected from the same centre of perspective, i. e. that nodal point of the projecting lens relating to the plate. A second and preferably enlarged photograph is now taken of the rectified image of the screen |25 in a direction at right angles to the screen by means of a lens |26 on a plate |21. The resultant photographic image will be free from distortion errors due to the oblique projection and also from errors due to lack of flatness in the rst plate.

In the arrangement of Figure 5, the illuminating device is generally similar to that of Figure 3, except that the knife edge |2| is replaced by a narrow slit |30, so that the surface of the object is illuminated by a single thin line of light running along the section profile instead of by a band of light having a sharp edge in the sec-w` tion plane. This gure also shows a projecting device giving directly a rectied image free from distortion due to the oblique projection. This device comprises a wide angle projecting lens I3| which has its optical axis perpendicular to the section plane but is displaced to one side of the prole so that the rays pass obliquely through the lens. This lens projects an image of the profile on to a projection screen or photographic plate |32 lying parallel to the section plane. This arrangement, which can be such as to give an enlarged image on the screen, if desired, operates satisfactorily provided that the direction of projection does not make too large an angle with the perpendicular to the section plane. Fairly good results can be obtained when this angle is 40 or less, but for larger angles an lappreciable amount of distortion is present in the image with the types of wide angle lens at present available.

Such diniculties are obviated in the preferred oblique projecting device illustrated in Figure 6, which is such as to give an image free from all 3 lens.

distortion due to the oblique projection. This projecting device comprises a projecting lens |40 and two collimating lenses |4|, |42 spaced apart symmetrically one on either side of the projecting lens. The projecting lens |40 itself is preferably of the anastigmat or rectilinear type and may consist of a divergent component disposed between two convergent components symmetrically arranged, with its nodal points |43, |44 in the air gaps between the components.

The two collimating lenses |4|, |42, each of which may consist of an achromatised doublet, are of equal focal length, and the principal focus of each lens coincides with the nodal point |43 or |44 of the projecting lens relating to the space in which the collimating lens lies. Each collimating lens is spherically corrected with respect to such nodal point, and fulls the sine condition within the angle subtended by the projecting@ to compensate for the aberrations, particularly astigmatism and curvature of field introduced The projecting lens |40 may be correct by the collimating lenses.

The whole system is symmetrical and produces an inverted image at unit magnification, the object and image planes |45, Iflt` being equally inclined to the optical axis. Slight inequalities in the powers of the collimating lenses may, however, be compensated for by a slightly asymmetrical arrangement, in which the central lens |40 is displaced towards the weaker of the two collimating lenses.

With this arrangement the principal rays froml points in the object plane |45 (which will usually be constituted by the section plane) to corresponding points in the image plane |46 are parallel to the optical axis in the spaces outside the collimating lenses, and it will be Clear that this -condition ensures that a rectified image free from distortion due to the oblique projection will be obtained. Such an image can be satisfactorily enlarged by means of ordinary enlarging apparatus having its axis perpendicular to the image plane, provided that the surface of the plate or screen on which the image is projected is a true flat surface.

It is not essential to the system to employ a projecting lens between the two collimating lenses, and such lens can be omitted altogether if the object and image planes pass respectively through the foci of the two collimating lenses. The provision of the central lens,.ho\vever, has the advantage of bringing the two planes closer together, and this in turn facilitates correction of the aberrations.

This arrangement has the property (when unit magnification is employed) that the object and image planes can be moved equal distances in the same direction relatively to the lens system with in limits determined by the positions of the collimating lenses, without disturbing either focussing or magnification. The system will also act to remove substantially all distortion due to oblique projection. In view of these advantages it will usually be preferable to employ unit magnication in the manner described, but it will be appreciated that in some instances it will suce to remove only keystone distortion (i. e. the distortion such that a square in the object plane with two sides parallel to the line of intersection of the object and image planes will be imaged as a trapezoid in the image plane), and to depart from unit magnication leaving rectangular distortion in the image (i. e. the distortion such that the said square will be imaged as a rectangle iwi t gesamt with two sides parallel to the line of intersection of the two planes). This may be done by using collimating lenses of different focal length or by displacing the Icentral lens from the centre of the system so that it adds to the power of one of the collimating lenses, provided that afocal adjustment is maintained. In this case the necessary angular relationship to ensure accurate focussing at the desired magnification is that the ratio of the tangent of the angle between the object plane and the optical axis to that between the image plane and the optical axis should be equal to the magnification of the system. As has been mentioned, this modified arrangement will give an image suffering from rectangular distortion, which can, however, be compensated for by the use of an anamorphotic enlarging system, including one or more cylindrical lenses. Even without such compensation, a rectangular-ly distorted image may itself be adequate, for example in cases where the relative dimensions in the undistorted direction alone are of interest.

Figures 7 and 8 show modifications of the oblique projecting system of Figure 6, wherein a rectified image free from keystone and rectangular distortion can be obtained at magnications other than unity. This is achieved by the use of tilted lenses.

The simple arrangement of Figure '1 comprises a projecting lens |50 and two collimating lenses |52 of different focal lengthl one on either side thereof. The collimating lenses are so spaced from the projecting lens that their principal focal points are approximately coincident with the nodal points of the projecting lens. Each collimating lens is mounted square to the optical axis of the system, i. e. with its optical axis coincident with such axis, and the projecting lens is tilted at such an angle lto the axis that the object, which lies in a plane |53 inclined to the optical axis, is imaged in a plane |54 equally inclined to the axis.

The angle of tilt of the projecting lens |50 necessary to produce this result, can best be explained by iirst considering the conditions which would arise if `the projecting lens were not tilted (as indicated in dotted lines). In such a case the plane |55 containing the first virtual image (i. e. the virtual image of the object formed by the first collimating lens |5|) and the plane |56 containing the second virtual image (i. e. the image of the first virtual image which is formed by the projecting lens |50 and from which the final real image |51 is formed by the second collimating lens |52) intersect in the nodal plane of th-e projecting lens. Tilting of the projecting lens about its nodal point will not substantially alter the position of the point of intersection of the second virtual image |56 and the optical axis, and the effect of such tilting will be to rotate the second virtual image about such point of intersection into a plane |58 at the same inclination to the optical axis as the plane |55 of the first virtual image, thereby tilting the final real image from the plane |51 into the plane |54 which is inclined to the optical axis at the same angle as the object plane |53, the proportions in the object and image yspaces preferably being geometrically similar to one another.

In practice the arrangement of Figure '1 will require modification in order to provide correction for spherical and other aberrations. For this purpose the projecting lens |50 is preferably of the anastigmat or rectilinear type and may consist of a divergent component between two convergent components symmetrically arranged, with its nodal points in the air gaps between the components. In view of the tilting of the lens, the two parts of the optical axis of the system on opposite sides of it,V will be parallel to one another respectively through the two nodal points, and each part of such axis Will coincide with the optical axis of the corresponding collimating lens. The collimating lenses, which may each consist of an achromatised doublet, and the projecting lens are preferably corrected in a manner similar to that described with reference to the arrangement of Figure 6. Since with this arrangement the principal rays in the object and image spaces are parallel to the optical axis, it will be clear that a rectified image free from distortion will be obtained.

4This arrangement may be modified by tilting the collimating lenses, either instead of or as Well as the projecting lens. In such a modification the two collima-ting lenses should be tilted through the same angle in order to avoid keystone distortion, and in one convenient arrangement, as shown in Figure 8, all three lenses |60, |6I, |62 are equally tilted. In this case, the collimating lenses IGI, |62, if tilted through an appreciable angle, should have the form of anastigmatic lenses, special regard being paid to the correction of the aberrations within the field subtended by the projecting lens. The focal lengths of the collimating lenses are measured normally to their nodal planes from the corresponding nodal points of the projecting lens, or in other words their principal focal points lie in the corresponding nodal planes of the projecting lens. The nodal point of the projecting lens corresponding to a collimating lens need not be the nodal point nearer to such lens, and'in the example llustrated the nodal points are crossed. The arrangement of Figure 8 has the advantage that appreciable magnification can be obtained without excessive tilting of the lenses, the object and image planes |63, |64 being again equally inclined to the optical axis of the system which is displaced parallel to itself in its passage through each lens owing to the separation of the nodal points of the lens.

The foregoing rectifying oblique projecting systems are suitable for projecting the image of the section proiile on to a photographic pla-te or a projection screen, but not for direct viewing of the profile through an eyepiece owing to the fact that the image plane is inclined to the optical axis. The systems can, however, be extended to suit direct viewing in a variety of Ways and Figures 9-12 show four alternative arrangements of this kind.

In the arrangement of Figure 9, a rectified image of the section profile |10 is obtained in an inclined image plane |1| by means of anoblique projecting lens system |12, |13, |14 arranged in the manner of Figure 6, 'and a further projecting lens |15 ac-ts to produce a further image of such rectified image in a plane |16 square to the optical axis of the whole system, so that such iinal image can be directly viewed through an eyepiece |11. It will be appreciated, however, that the final image, although accurately focussed in.

a plane square to the axis, will be distorted, and in order to enable accurate measurements to be made on the image, notwithstanding such distortion, a graticule is provided in the iirst image plane |1 I so that the profile and the graticule are equally distorted in the final image. Since it would in practice be rather disturbing to attempt to make measurements on an image suffering from keystone distortion, it is preferable to provide a collimating field lens |18 between the first image plane Ill and the projecting lens |15, so that the final image will suffer only from rectangular distortion. The provision of the collimating field lens |18 is also convenient in that it contributes somewhat towards the formation of the final image and can be corrected in association with the tilted projecting lens |75.

Figure shows a modification of the arrangement of Figure 9, in which the rectifying projecting lens system |12, |13, |74 is omitted, the superimposition of the graticule on the section profile in the observed image being obtained by means of a semi-transparent inclined reflector |80 disposed between the section plane and the tilted projecting lens |75. The graticule is disposed in a plane |8|, which is the image of the section plane in the reiiector |80, and is illuminated by a source of light |82 associated with a condenser |83. If the collimating field lens |78 is employed, it may be located between the reflector and the section plane, in which case a second and similar collimating eld lens |84 should be provided between the reflector and the graticule. This arrangement has the advantage over the arrangement of Figure 9 that it reduces the number of images and correspondingly improves the definition of the final observed image, which however is still a distorted image.

Figure ll illustrates a more complicated system for producing a rectified image for direct viewing. 4In this example a rectangularly distorted image of the section plane 19t] is 0btained square to the optical axis by means of a tilted projection lens ISI in conjunction with a collimating eld lens |92 in a manner analogous to that described with reference to Figures 9 and 10, and a sphere-cylindrical field lens |93 is disposed at the image point. The rectangular distortion in the image is then corrected by means oieafiamorphotic system comprising a central objective |94 having spherical surfaces and two cylindrical lenses |95, |96 one on either side of the objective |9d and equally spaced therefrom. The two cylindrical lenses have their axes at right angles to one another and the equivalent focal lengths in the two meridians arevthe same, so that the images formed in the two meridians are coplanar, but the nodal planes are displaced in opposite directions with the result that the system magniiies in one direction and reduces inversely in the other direction. By making the magnification equal to the square root of the ratio of the sides of the distortion rectangle to be corrected, a true rectified image will be obtained for viewing through the eyepiece |91. A graticule |98 may be provided in the final image plane.

A simpler arrangement for obtaining a rectified image which can be viewed through an eyepiece is shown in Figure l2. In this example a rectified image of the section profile 200 is obtained on a suitable diffusing surface 20| by means of an oblique projecting system 262, 203, 204, similar to that of Figure 6, and this image is directly viewed through an eyepiece 205.

It will be appreciated that the direct vision systems of Figures 9-12 can all be modified, if'

desired, for enlarged projection on to a screen by providing a suitable projecting lens of wide aperture in place of the eyepiece.

It will be appreciated that the alternative forms of illuminating device shown in Figures 1-5, as also the forms now to be described with reference to Figures 13-21, can be interchanged with one another, as may be desired, for use with any chosen projecting or direct vision device.

The illuminating devices so far described are more especially suitable for use with objects for which the section profile does not diverge to any considerable extent from a straight line. For precision measurement, for example, of screwthreads or gear teeth, however, it will usually be preferable to take into account the shape of the surface of the object and to provide an illuminating device especially suited to such shape in order the more sharply to define the section profile, and Figures 13-21 illustrate a number of alternative forms of illuminating device for this purpose.

Figure 13 shows an illuminating device formed in two parts each associated with a straight line (or approximately straight line) portion of the section prole, the two portions of the profile being at an angle to one another. In the example illustrated the two portions in question are assumed to be the two flanks 2|U, 2|| of a tooth of a thread or gear wheel. The illuminating device comprises two objectives 2|2, 2|3 respectively for focussing two slits or knife edges 2|ll, 2|5 on the flanks 2H), 2li of the tooth, the slits or knife edges being illuminated by sources of light (not shown) arranged in the manner described with reference to Figure 3 or Figure 5. The section plane is in this instance the plane of the paper and the two slits or knife edges both lie in the plane parallel to the corresponding tooth iianks. The optical axis of each objective will be perpendicular'to the appropriate nank, if practicable, but more usually it will be necessary for it to be inclined at a small angle to such perpendicular, as shown, the objective being in the form of a wide angle lens.

When the available direction of illumination is steeply inclined to the perpendicular to the tooth flank, it'will not be practicable to employ a wide angle lens, and in such a case the slit or knife edge should be inclined to the tooth flank with the nodal plane of the objective passing through the intersection of the slit or knife edge with the ank. Such an arrangement is shown in Figure 14, where the two slits or knife edges 2|6, 2|1 are in a straight line with one another (and may be continuous with one an other) and are focussed on the tooth flanks 2|0, 2|| by means of appropriately tilted objectives 2|8, 2|9.

Figures 15 and 16 show a further variant suitable for illumination of a number of noncollinear parts of the profile such for example as a series of teeth of a thread 220. In this instance it is not essential for the parts of the profile to be straight lines. This arrangement employs a knife edge 22| shaped approximately to suit the shape of the prole, an image of the knife edge, illuminated by a source 222 and condenser 223, being focussed by a single optical system on the surface of the object. Since the individual parts of the knife edge and profile are inclined to the optical axis of the system, it is preferable to employ a rectifying oblique projecting system to ensure accurate focussing on the .object surface, and to this end the optical system 224, 225, 226 is arranged in the manner already described with reference to Figure 6. Some slight degree of adjustment of the central lens 224 along its optical axis is permissible to vary the magnification in accordance with any error, for example in pitch, in the object. In the arrangement shown in .Figures l and 16 the knife edge 22| is made by bending sheet metal to the required shape and cutting it off in a plane. It will loe appreciated, however, that the knife edge can be made in other ways, as for example by milling from the solid. Figures 17 and 18 illustrate such a modification, wherein a knife edge 221 is obtained by cutting a solid counterpart of the object, for example a nut if the object is a screwthread, in the appropriate plane and polishing the cut surface. This polished surface acts as a reflector to replace the portion of the cone of light, which is cut off by the solid body of the cut counterpart and whose loss would tend to cause an unsatisfactory image of the knife edge on the surface of the object.

It will be appreciated that a knife edge shaped, as in the arrangements of Figures -18, to suit the shape of the section profile may be employed in the shadow illumination arrangementV of Fig`- 'ure l in place of the simple straight knife edge |05 therein and will give more satisfactory definition of the profile.

Figures 19-21 show a preferred form of illuminating device for use for such purposes as examining gear teeth or screwthreads, by means of which the two flanks of a tooth can be simultaneously illuminated, the device being adjustable to suit teeth of different angle and size. In this arrangement two knife edges 230, 23| constituted by the two sides of a narrow slot cut in a flat piece of metal are illuminated by a source 232 and condenser 233 andare focussed by means of an objective 234 in conjunction with a collimating lens 235 and suitable plane reflectors on the two flanks 236, 237 of a tooth on the object. The reflectors are symmetrically arranged with respect to a plane, which passes through the optical axis at right angles to the section plane and which also passes through the slot midway between thc two knife edges 230, 23|, and may be constituted by the faces of a prism or prisms or by separate reflectors. It will be assumed for convenience of description that the section plane and the plane of symmetry are both vertical (so that Figure 19 is a horizontal section and Figure 20 a vertical section whilst Figure 2l shows the reflectors in end elevation) although it Will be appreciated that the device may in practice be otherwise arranged.

The rays from the knife edge 230 are reflected at a vertical refiecting surface 23B, inclined to the optical axis so that they pass out generally horizontally at right angles to the optical axis, and are then reected downwards and inwards by a further reflecting surface 239 todeflne the part 240 of the section profile on the tooth flank 235. The rays from the other knife edge 23| are similarly reflected, on the other side of the optical axis, at surfaces 24| and 242 to define the part 243 of the profile on the tooth flank 23T. In order to prevent confusion which might otherwise occur from the imaging of both knife edges on each tooth flank, a mask 244, which may consist of a vertical wire, is disposed between the objective 234 and the reflectors in order to soften the definition of the undesired image without affecting that of the desired image 240 or 243. A similar effect could alternatively be obtained by suitably chamfering the edges of the reflecting prisms. The two knife edges will thus be accurately focussed on the tooth flanks to define the section profile, provided that they occupy the correct position and inclination with respect to the optical system The vertical chain line 245 in Figure 2l through the point of intersection of the two images 240, 243 is itself the image of the oblique chain line 246 in Figure 20 through the base of the slot forming the knife edges. It will be clear therefore that if the slotted plate is moved parallel to itself along the line 245, the point of intersection of the images 24U and 243 will move along the line 245, the images remaining parallel to themselves, and such translational adjustment of the plate will thus constitute the adjustment necessary to suit a tooth of the. same angle but of different size. In a similarway rotation of the plate about the base of the slot, where it is intersected by the line 245, will have the effect of `altering the inclination of the two images 24), v243 to one another without moving their point of intersection, thus accommodating teeth of different angle. The device can thus be adjusted to any required angle or size of tooth having straight flanks. Usually the flanks of a tooth `--will be straight or very nearly so, so that depth of focus will adequately cover slight divergences, but for special cases where the flanks are con-- siderably curved, the knife edges may be likewise curved (in the plane of the paper in Figure 20) whilst remaining parallel to one another.

Difficulty arises in many instances in connection with the projection of the section prole from the fact that it is impossible to view the Vwhole profile from any one viewpoint owing to obstruction of the view by other parts of the o-bject under examination. Figures 22-34 show a variety of `arrangements in which this dilculty is avoided by mounting the projecting device so that it can be rotated to enable the profile to be viewed from separate viewpoints, the arrangements being such that the separate partial images of the profile thus obtained are 'accurately correlated with one another. The arrangements are especially useful when the profile image is received photographically, for it then becomes possible to build up the complete profile by two or more exposures of the same plate taken in different positions of the projecting device.

In one arrangement, illustrated in Figure 22, the complete projecting device 250, 25|, 252, which is of the kind described with reference to Figure 6, together with a photographic plate 253 in the image plane, is mounted for rotation within suitable angular limits about an axis indicated in chain line at 254 suitably inclined to the optical axis of the projecting device. Conveniently the axis 254 is at right angles to the optical axis and at 45 to the section plane 255 and passes through the intersection of the optical laxis with the section plane. In the normal position of the apparatus, as shown, the section plane, the image plane and the plane containing the optical axis and the axis of rotation are mutually perpendicular, so that in this position the image plane is inclined at 45 to the optical axis. In order, however, to maintain the image stationary and sharply focussed on the plate 253 during the rotation, it is necessary for the plate itself to be rotated about a secondary axis relatively to the optical axis during the rotation of the optical axis about the main axis of rotation 254. This can be achieved by employing for this moving secondary axis a line 256 perpendicular to the optical axis through the point of intersection thereof with the image plane, the rate of rotation of the plate about the axis 256 (relatively to the optical axis) being made equal to that of the system about the main axis 254 by means of suitable interconnecting gearing. With this arrangement a photograph of part of the prole can be taken in one position of the system and then a photograph of another part by exposure of the same plate in a new position, and so on until a composite photograph of the complete profile has been obtained. The illuminating device shown by way of example as of the kind described with reference to Figure 3 is indicated at 257.

This arrangement can be modified by employing a main axis of rotation not at right angles to the optical axis. In this ease the secondary axis will be coplanar with the main axis and will be inclined to the optical axis at the same angle as the main axis. Thus when the main axis is at right angles to the section plane, the secondary axis will be at right angles to the plate, and when the main axis lies in the section plane the secondary axis will lie in the image plane.

It will usually be more convenient, however, to employ a stationary photographic plate, and this can be achieved in a variety of ways by incorporating reecting devices intgthe rotating optical system.

In one such arrangement, shown in Figure 23, the photographic plate 260 is xed at right angles to the section plane 26| and the axis of rotation 262 passes through the plate and the section plane at 45 to each. The optical system includes, in addition to the three lenses 263, 264, 265, two plane reflectors 266, 267 each of which deects the optical axis through a right angle, so that the optical axis intersects the section plane and the plate in the same points as the axis of rotation. The illuminating device is indicated at 268.

This arrangement may be modified, if desired. as shown in Figure 24 by turning the plate 260 through a right angle so that it is parallel to the section plane 26|, and adding an additional reflector 269 at one of the corners. In this way an image erect in a direction parallel to the plane of the drawing and inverted in a direction at right angles thereto is obtained to suit the relative directions of rotation of the section plane and plate with respect to the optical axis.

In another arrangement, shown in Figure 25, the plate 270 is coplanar with the section plane 27| and the axis of rotation 272 also lies in such plane. The optical axis of the projecting lens system 273, 274, 275 leaves the section plane at 45, is deflected by a reflector 276 back again parallel to such plane, and isdeflected again by another reiiector 277 to meet the plate at 45. The image in this case is completely inverted. The illuminating device is shown at 278.

In the arrangement of Figure 26, the plate 280 is parallel to the section plane 28| with the axis of rotation 282 perpendicular to both, the optical axis of the lens system 283, 284, 285 intersecting both planes at 45 with two intermediate reflections at 286 and 287. The image is again completely inverted.

In all these arrangements it has been assumed that the illuminating device will adequately illuminate the whole section proiile, and it will be realised that in some instances more than one illuminating device or alternatively a movable illuminating device will be required to illuminate the particular part of the profile being projected. 75

The arrangement of Figure 26 has the advantage that a single illuminating device 288 can be employed which is movable with the projecting device, since the axis of rotation 282 is perpendicular to the section plane.

The requirements as to erection or inversion of the image depend on the relative directions of rotation of the section plane and the plate with respect to the optical axis, and arrangements other than those above mentioned may be employed. Since it is more convenient for rectifying purposes for the lens system itself to give an inverted image, the requisite further inversions in individual cases are provided by the reflectors, and in some instances special reectors such as Doves prisms or roof prisms are required.

Thus in the case (shown in Figures 2'7 and 28) of a stationary plate 290 perpendicular to the section plane 29| with the axis of rotation 292 at to both, the optical axis of the -lens system 293, 294, 295 may leave the section plane at right angles to the axis of rotation 292 and after crossing such axis may approach the plate at right angles to the axis. In this case it is necessary for the image to be inverted in a direction parallel to the plane of the drawings and erect in a direction at right angles thereto. This is achieved by providing a roof prism 296 at one of the corners and a plane reflector 297 at the other corner. A similar result can also be obtained as shown in Figures 29 and 30 by replacing the roof prism 296 by a plane reflector 298 and inserting a Doves prism 299 at a convenient point in the ray path.

In the arrangements of Figures 22-30 the desired rectication of the image has been obtained by means of the lens system of Figure 6, but other rectifying arrangements may be employed, if desired.

Thus Figures 31 and 32 show an arrangement in which a wide angle lens 300 is employed, the section plane 30| and the photographic plate 302 being parallel to one another and to the nodal plane of the lens. In this case the lens 300 and the plate 302 are mounted on a frame 393 rotatable about an axis 304 at right angles to the section plane. In order to correlate the partial images with one another in this case it is necessary for the plate, during its rotational movement about the axis 304, to remain in the same orientation in its plane. This can be achieved by a simple parallel link mechanism, consisting in the example illustrated of a link 365 connecting a point on the plate carrier 306 (which is itself rotatable in the frame 303) to a fixed point, the dimensions of the parts being such that the link 305 always remains parallel to the line joining the main axis of rotation 304 with the secondary axis of rotation of the plate carrier 306.

Figure 33 shows an alternative arrangement using a wide angle lens 3|0, wherein by the use of reflectors 3H, 3|2, which are rotatable with the lens 3H) about an axis 3|3 intersecting the section plane 3|4 and the plate 3|5 at right angles in points which are images of one another, it becomes possible to employ a stationary plate.

Figure 34 illustrates an arrangement in which a rectifying projecting system similar to that of Figure 2 is mounted rotatably. In this example the section plane 320, the plate 32| and the intermediate diffusing screen 322 are all parallel to one another. The two projecting lenses 323, 324, and two pairs of reflectors 325, 326 and 327, 328 

